In mass production the workpieces, for instance small-arms ammunition, are arranged in a single row by rotary conveyors, formed with spaced seats, adjacent to working stations set up to act sequentially on the workpieces. As is known, a rotary conveyor takes a workpiece into one of its seats at a location along its periphery and transfers it, at another location, to another such rotary conveyor or to a working station. In addition a workpiece can move from a working station to a rotary conveyor to go toward another working station or to a receptacle. The essential advantage of mass production is to increase the production rate while reducing costs. Nonetheless the continuous movement of the workpieces poses delicate timing and testing problems.